Getting rid of Johnson grass takes repeated control over multiple seasons since rhizomes and seeds keep it coming back.
For the full breakdown, see our best Herbicide For Johnson Grass guide.
Johnson grass (*Sorghum halepense*) is one of the toughest weeds on a property because it spreads two ways simultaneously: seeds that scatter in the wind and rhizomes that creep underground. There’s no single spray-and-done fix. The winning approach combines stopping seed production, digging out rhizomes, and repeating your efforts across the growing season. Here’s the practical sequence that actually works.
Why Johnson Grass Keeps Coming Back
Johnson grass reproduces through both seeds and rhizomes, the underground stems that send up new shoots. Even a small broken piece of rhizome left in the soil can sprout a whole new plant. Meanwhile, a single mature plant can drop thousands of seeds that stay viable in the soil for years.
The Missouri Department of Conservation breaks the control strategy into four steps: prevent seed spread, destroy seedlings before rhizomes form, weaken and kill existing rhizomes, and control new infestations as they appear.
Mechanical Control: Digging, Mowing, and Tillage
For small patches, hand-pulling works best after rain when the soil is soft. Dig out the entire plant including all rhizomes, and remove every plant part from the site. Broken rhizome fragments will regrow, so take your time and get the full root system.
For larger infestations, repeated mowing through the growing season prevents seed production and slowly weakens the rhizomes. The common mistake is mowing once and stopping — that just makes the grass bushier and stronger.
Herbicide Options That Work
For dense patches, A tractor-mounted power sprayer works for larger areas.
When Johnson grass grows among wanted vegetation, spot treatment beats broadcast spraying every time.
The Realistic Timeline for Full Control
Johnson grass control is long-term management, not a one-time fix.
| Method | When to Apply | Key Detail |
|---|---|---|
| Hand-digging | After rain, soil soft | Remove all rhizomes; fragments regrow |
| Repeated mowing | All season | Prevents seed set, weakens rhizomes |
| 2% glyphosate spray | 18″ tall to early flower | Non-selective; protect neighbors |
| Seedling treatment | May, 2–8″ tall | Catch them before rhizomes form |
| Fall plowing | After first frost | Exposes rhizomes to winter kill |
The biggest mistakes come down to stopping too early: leaving rhizome fragments, treating only the tops, letting seeds form, or applying once and walking away. Each season you skip control, the seed bank and rhizome system grow stronger. Stay consistent across two to three seasons and the infestation shrinks dramatically.
FAQs
Does mowing kill Johnson grass?
Mowing alone won’t kill established Johnson grass, but repeated mowing through the growing season prevents seed production and gradually weakens the rhizome system.
What kills Johnson grass permanently?
No single treatment permanently eliminates Johnson grass because both seeds and rhizomes regenerate. The most effective approach combines preventing seed set, digging out rhizomes, and applying labeled herbicides at the right growth stage.
Is Roundup effective on Johnson grass?
Yes, when applied correctly. Spot-treating invading plants also works. Since glyphosate is non-selective, shield desirable vegetation nearby and always follow the product label instructions.
References & Sources
- Missouri Department of Conservation. “Johnson Grass Control.” Details four-step strategy: prevent seeds, destroy seedlings, weaken rhizomes, control new infestations.
- Purdue University Extension. “Johnsongrass Control in Conservation Plantings.” Provides herbicide timing and rates for seedlings and established plants.